Gonadotropin Releasing Hormone (GnRH) is now the best available biotechnological tool for the induced breeding of fish. GnRH is the key regulator and central initiator of reproductive cascade in all vertebrates.
It's a decapeptide and first isolated from pig and sheep hypothalami with the ability to induce pituitary release of luteinising hormone (LH) and follicle stimulating hormone (FSH). Since then only one form of GnRH has been identified in most placenta! mammals including human beings as the sole neuropeptide causing the release of LH and FSH.
However, in non-mammalian species (except guinea pig) twelve GnRH variants have now been structurally elucidated, among them seven different forms have been isolated from fish species. Depending on the structural variants and their biological activities number of chemical analogues have been prepared and one of them is sahnon GnRH analogue profusely used now in fish breeding and marketed commercially throughout the world. Question is why fishes require an induction by this neuropeptide hormone to release germ cells from male and female partners so that fertilization of oocytes by sperms may occur in the aquatic ambience.
In fact, most of the economically important culturable fish in land-locked water do not breed until they are induced by the hormone. Fish in marine or riverine water do not need induction by hormone for their breeding. But to culture fish with the intention of increasing the yield of production in either ocean or river is not possible. Culture requires a controllable area, that's why pisciculture to enhance the yield is restricted to land-locked aquatic bodies where fish do not usually breed or spawn without the induction of hormone.
Since fish meat has been found to be the best quality food among all animal meats, there is an enormous increase in fish producing industries in global scenario recently where the major rate-limiting factor is breeding. The induced breeding of fish is now successfully achieved by the development of GnRH technology. In this overview, a brief description is given to conceive the biotechnology of this highly important area of our food supplement.
Structure of GnRH—GnRH was first isolated from the mammalian hypothalamus as a decapeptide(Burgus et al., 1972; Mastuo et al., 1971). Since then, several structural_variants of GnRH have been described which can be seen in FIG. 1 (shown under the heading brief description of accompanying drawings).
There has been a striking conservation in GnRH peptide length, the first four —NH2 terminus and two —COOH terminus amino acids are remarkably conserved in different vertebrates. Whatever changes occurred are between 5 and 8 amino acid residues, position 8 is most variable followed by position 6,5 and 7. This highly variable position 8 suggests its role in the variation of biological activity between the species and its critical function in recognizing GnRH receptors on the pituitary cell membrane. To exert its biological function it has to first recognize its receptor on the pituitary gonadotroph cell membrane and when it has to do so its linear structure changes into a loop. The NH2- and COOH-terminal domain of GnRH are closely opposed when GnRH binds to its receptor and this is the proposed result from a P-II type rum involving resides 5-8. The P turn creates a hairpin loop that aligns N and C termini (Sealfon et al., 1997) (FIG. 2). (Shown under the heading brief description of accompanying drawings).
GnRH from Indian fish—Although India has several varieties of indigenous culturable bony fish, practically nothing is known about their GnRH. This is really a very unfortunate situation for which we still have to depend on the supply from other countries to investigate the reproductive biology of our economically important culturable fish as GnRH is the key regulator of reproductive control mechanism.
Several years ago we initiated this research and faced serious problems. For the isolation of this peptide we would need an assay system to identify the desired molecule at each step of purification. First piscine GnRH isolated purified and structure determined was Salmon GnRH (Sherwood and her associates in 1983), and the most current GnRH purified and characterized by two different groups Carolsfeld et al and Robinson et al., was in 2000. Between the span of this time, number of GnRH variants have been isolated by depending on the immunoreactivity of this molecule. During Salmon GnRH isolation, Sherwood et al took the help of anti-GnRH antibody from mammalian source, later they developed Salmon anti-GnRH antibody which permitted to isolate different GnRH forms from other sources.
Meanwhile Jamaluddin et al, (Gen comp Endocrinology May 1989; 74(2): 190-8) of applicants laboratory developed a very dependable GnRH bioassay. Basic principle of this bioassay is to get the release of GTH from murrel primary pituitary cell culture and determine the amount of GTH released by GTH-RIA.